The present disclosure relates to integrated optical circuits. More particularly, the present disclosure relates to the stabilization of resonant optical elements in integrated optical circuits.
The silicon microring resonator has gained significant attention for use in an energy-efficient and high-bandwidth photonic system and is ideally suited for both inter- and intra-data center communication. The small footprint of the resonator (a few hundred μm2) allows multiple microring resonators to be cascaded on a single bus to form an elegant and compact wavelength-division-multiplexed system. Compared to other non-resonant modulators, the efficiency of the microring resonator is dramatically enhanced owing to its resonant nature. However, for the same reason, the microring resonator is susceptible to thermal fluctuation that can cause an undesired resonance variation due to the strong thermo-optic effect present in silicon waveguides. For example, in a highly clustered system, a transient thermal load in an adjacent channel or a slow ambient temperature drift can lead to a microring resonator modulator failure. Therefore, resonance control is necessary to insure the stability of the microring resonator modulator in a real-word deployment. Several methods have been reported in the literature aimed at an integrated solution for resonance control. The most common approach uses a power detector and an integrated heater that can form a closed-loop via proportional-integral-differential (PID) feedback. Other non-PID based approaches exist such as the homodyne method.
FIGS. 1 and 2 illustrate some feedback-control geometries that have been employed for the stabilization of a microring resonator modulator. In FIG. 1, the feedback signal (i.e., photocurrent) is provided by a Ge photodetector at the drop-port of the microring resonator. For a stable single wavelength input, the photodetector monitors the power variation owing to the resonance change of the microring resonator. The PID feedback-control is then applied to compensate for drift using the heater power for stabilization. In FIG. 2, a similar control method is presented, with the exception that a designated photodetector placed inside the ring is employed to measure the resonant power in order to provide the feedback signal.